The present invention relates to servo-driven clamping units for use in injection molding machines, and more particularly, to a servo-driven clamping unit for use in an injection molding machine, which can facilitate precise positioning control of mold clamping, so as to improve stability and reliability for proceeding an injection molding process.
Generally, a conventional injection molding machine adopts a hydraulic clamping mechanism for performing mold-opening and mold-closing operations, and is mainly composed of a machine base, a clamping unit, an injection unit and a hydraulic electric control system. FIG. 1 illustrates a conventional clamping unit 1, comprising: a front platen 3; a rear platen 4; a plurality of tie bars 2 symmetrically positioned and mounted in parallel to corners of the platens 3, 4 respectively; a movable platen 5 movably sleeved about the tie bars 2 between the front platen 3 and the rear platen 4; a hydraulic cylinder 6 mounted on the rear platen 4, for providing power to drive the movable platen 5 to move forward and in reverse along the tie bars 2; and a set of toggle linkage mechanisms 7 for interconnecting the rear platen 4 and the movable platen 5, and driven by the hydraulic cylinder 6 for inducing the back and forth movement of the movable platen 5. This conventional injection molding machine, however, is endowed with quite a few of drawbacks. First, the hydraulic clamping mechanism is not sensitive enough, thereby making the use of the hydraulic cylinder 6 for operating mold-opening or mold-clamping, not able to achieve high accuracy in mold positioning control. Further, hydraulic transmission may be affected by many factors. For example, variation of temperature or humidity in a working environment influences viscosity of hydraulic oil and frictional resistance of oil movement in the hydraulic cylinder 6; therefore, in practice, the hydraulic clamping mechanism is often defective with time-ineffective and energy-consuming problems. Moreover, the hydraulic cylinder 6 acting as a power source for mold clamping operation, is formed with a guide rod connected to one end of a connector device 8, whereas the other end of the connector device 8 is pivotally coupled to the toggle linkage mechanism 7. However, during element assemblage, a piston ring in the hydraulic cylinder 6 can bear with a certain extent of flexibility, thereby making the hydraulic cylinder 6 not possibly made in perfect association with the connector device 8; this therefore degrades the dimensional accuracy of fabricated products. In addition, this hydraulic system cannot efficiently control the entire progress of moving the guide rod of the hydraulic cylinder 6, so that precise positional control during mold-clamping operation is not possibly achieved by using the hydraulic system.
U.S. Pat. No. 4,642,044 discloses a conventional servo-driven clamping unit 1xe2x80x2 for use in an injection molding machine disclosed, so as to solve the foregoing problem of positioning control, as shown in FIG. 2. The clamping unit 1xe2x80x2 adopts a servo-motor 9 to drive mold-opening and mold-closing operations, wherein the servomotor 9 is mounted on a rear platen 4xe2x80x2, and turns to generated torque force that is transmitted through a shaft connector 10 associated with the servo-motor 9, an idler gear 11 and a follower gear 12 engaged with the idler. When the follower gear 12 is driven to rotate, a ball screw 14 coupled to the follower gear 12 by means of a guide device 13 can be induced to move forward and backward along a horizontal direction, allowing the torque force to be converted into axial pushing force. The ball screw 14 is further connected to a toggle linkage mechanism 7xe2x80x2 by means of a connector device 8xe2x80x2, wherein the toggle linkage mechanism 7xe2x80x2 interconnects the rear platen 4xe2x80x2 and a movable platen 5xe2x80x2. When the ball screw 14 moves forward, the toggle linkage mechanism 7xe2x80x2 is driven by the connector device 8xe2x80x2 to move the movable platen 5xe2x80x2 forward along tie bars 2xe2x80x2 and toward a front platen 3xe2x80x2, until mold halves being coupled together, so that mold-clamping operation is completed, and the servo-motor 9 stops operating at this time. For opening the mold halves, the servo-motor 9 turns at an opposite direction, allowing the ball screw 14 to move backward and generate a backward pulling force to move the movable platen 5xe2x80x2 away from the front platen 3xe2x80x2, so that the coupled mold halves can be separated apart from each other, and mold-opening operation is accomplished.
However, the foregoing servo-driven clamping unit 1xe2x80x2 has the following drawbacks. First, power transmission through the idler gear 11 and the follower gear 12 cannot be perfectly performed due to dimensional errors made during gear fabrication; this undesirably results in energy loss and also inaccuracy of positioning control during mold-clamping operation. And, the gear transmission system is further defective for noise production, frequent requirement of element lubrication, and reduction in usage lifetime of elements. Moreover, the ball screw 14 is assembled with the connected device 8xe2x80x2 by means of screws, and thereby easily applies unbalanced force to the connected device 8xe2x80x2; this may lead to rotational deviation of the ball screw 14 and detrimentally affect lifetime of the ball screw 14. Furthermore, two ends of the ball screw 14 are not balanced supported by only one guide device 13, and thus rigidity and stability of the ball screw 14 are not strong enough in operation, which in turn deteriorates the accuracy of positioning control during mold-clamping. In addition, the toggle linkage mechanism 7xe2x80x2 used in the conventional servo-driven clamping unit 1xe2x80x2 is relatively complicated in structure; this increases complexity in fabrication, and also makes the mold-opening or mold-clamping operation not time-effective to implement.
A primary objective of the present invention is to provide a servo-driven clamping unit for use in an injection molding machine, in which internal elements of the clamping unit are precisely engaged with each other for accurate power transmission and mold-clamping positioning.
Another objective of the invention is to provide a servo-driven clamping unit for use in an injection molding machine, in which two ends of a ball screw are well supported by bearing devices, allowing the ball screw to be accurately assembled in the clamping unit, so as to improve rigidity and stability of the ball screw in operation.
A further objective of the invention is to provide a servo-driven clamping unit for use in an injection molding machine, in which a toggle linkage mechanism is structurally simplified in element linkage, so as to enhance mold-clamping force.
In accordance with the foregoing and other objectives, the present invention proposes a servo-driven clamping unit for use in an injection molding machine, comprising: a plurality of tie bars symmetrically positioned and mounted in parallel to corners of a front platen and a rear platen; a movable platen movably sleeved about the tie bars and positioned between the front and rear platens; a servo-motor mounted on the rear platen, for providing power for operating the clamping unit; a belt-gear mechanism associated with the servo-motor, for transmitting torque force and turning speed provided from the servo-motor; a ball screw transmission mechanism including a ball screw and a guide device connected to the belt-gear mechanism, for converting the torque force transmitted from the belt-gear mechanism into axial pushing force, wherein the guide device is formed with a threaded hole for allowing the ball screw to be inserted through the threaded hole; a toggle linkage mechanism for interconnecting the rear platen and the movable platen, and bearing the axial pushing force from the ball screw transmission mechanism so as to drive the movable platen to move along the tie bars.
The invention is characterized of using the belt-gear mechanism, including an active gear, a passive gear and a belt; which belt is a timing belt, and is precisely toothed and engaged with the active gear and the passive gear. This perfect dimensional engagement therefore allows accurate power transmission, and eliminates the drawback of energy loss caused by incomplete engagement between conventional gears due to dimensional inaccuracy made in fabrication. Therefore, the use of the belt-gear mechanism is advantageous of reducing noise generation, efficiently transmitting mold-clamping force, as well as precisely controlling mold-clamping positioning.
In addition, the guide device of the ball screw transmission mechanism is sleeved about one end of the ball screw, and a connector device is affixed to the other end of the ball screw, with at least a bearing device being peripherally formed around the guide device and the connect device, respectively. This makes two ends of the ball screw well supported by the bearing devices, allowing the ball screw to be accurately assembled in the clamping unit, so as to improve rigidity and stability of the ball screw in operation. The guide device is further coupled to the passive gear of the belt-gear mechanism, and driven to rotate by the servo-motor in association with the belt-gear mechanism, so as to induce the ball screw to move forward and backward, and convert the torque force from the servo-motor into the axial pushing force, which is in turn transmitted to the toggle linkage mechanism for driving the movable platen to move along the tie bars.